Ferromagnetic films for high density recording and methods of production

ABSTRACT

A ferromagnetic film for magnetic recording comprises a copper substrate having particles of iron and iron oxides dispersed in the surface layer of the copper. The particles have maximum dimensions in the range between 50 and 500 Angstroms. The ferromagnetic film can be formed by ion implantation of iron ions into the copper substrate followed by heat treatment to permit growth of ferromagnetic particles to the desired size. As an alternative to ion implantation, the iron can be deposited on the copper substrate by sputtering or evaporation and mixed with the copper by ion beam mixing.

This application is a division, of application Ser. No. 561,692, filedDec. 15, 1983.

BACKGROUND OF THE INVENTION

This invention relates to ferromagnetic films and, more particularly, toferromagnetic films having extremely small ferromagnetic particle sizeand to methods for production of said films.

Ferromagnetic films provide an excellent medium for recording of video,audio and digital information. Magnetic tapes and disks are widely usedin the computer industry. Some computer users, such as Governmentagencies and insurance companies, require storage of huge amounts ofdata. To minimize the volume of the media for storing such quantities ofdata, high recording density is required. In the personal computerindustry, the need exists for low cost, high density recording media.

Present day magnetic recording media utilize iron oxide particlesdispersed in a polymer substrate. The iron oxide is in the form ofneedles 1 to 2 micrometers in length and is manufactured by a chemicalprocess. The iron oxide is imbedded in the substrate by flowing thepolymer over the surface and mixing in the particles. According to anewly developed technique, the iron oxide needles are arrangedvertically in the substrate in order to increase recording density.However, this technique is not compatible with existing recordingequipment. It has not been possible to produce iron oxide particlessmaller than the 1 to 2 micrometer needles described above, due tolimitations in the manufacturing process.

It is an object of the present invention to provide novel ferromagneticfilms.

It is another object of the present invention to provide novel methodsfor production of ferromagnetic films.

It is yet another object of the present invention to provide novelferromagnetic films for high density magnetic recording and methods ofproduction thereof.

It is still another object of the present invention to provide methodsof production of ferromagnetic films which achieve a high degree ofuniformity and control of parameters.

SUMMARY OF THE INVENTION

According to the present invention, these and other objects andadvantages are achieved in a ferromagnetic film comprising a substratematerial having low solubility for a selected ferromagnetic material anda plurality of particles of the selected ferromagnetic materialdispersed in the surface layer of the substrate. The particles have amaximum dimension in the range between 50 and 500 Angstroms. A preferredferromagnetic film includes particles of iron and oxides of irondispersed in a copper substrate.

According to another aspect of the present invention, there is provideda method for fabricating a ferromagnetic film. The method includes thesteps of providing a substrate of a material having low solubility for aselected ferromagnetic material and implanting ions of the selectedferromagnetic material into a surface layer of the substrate. Thesubstrate is then heated to permit growth of the particles of theferromagnetic material to the desired particle size. Typically, theparticles are permitted to grow to a maximum dimension between 50 and500 Angstroms when the substrate is copper and iron ions are implantedtherein.

According to yet another aspect of the present invention, there isprovided a method for fabricating a ferromagnetic film comprising thesteps of providing a substrate of a material having low solubility for aselected ferromagnetic material and depositing a thin layer of theselected ferromagnetic material on the surface of the substrate. Thelayer of ferromagnetic material is irradiated with an ion beam of aninert material so as to cause ion beam mixing of the ferromagneticmaterial into a surface layer of the substrate. The substrate is heatedto permit growth of particles of the ferromagnetic material to a desiredparticle size. The ferromagnetic material can be deposited by sputteringor evaporation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a ferromagnetic film incorporatingextremely small ferromagnetic particles which provide the capability forhigh recording density. As noted hereinabove, the ferromagneticparticles currently utilized in magnetic recording media areneedle-shaped particles of iron oxide (γ--Fe₂ O₃) having dimensions onthe order of 1 to 2 micrometers. Particles of this size can have morethan one magnetic domain. Magnetic particles can be characterized by acritical size below which only a single domain can exist. For iron, thiscritical size is approximately 200 Angstroms. Magnetic particles arealso characterized by a second critical size below which behavior issuperparamagnetic. That is, the magnetization of the particle can changedue to thermal effects when no field is applied, thereby rendering suchparticles unusable in recording media. For iron, this second criticalsize is approximately 50 Angstroms. The size at which ferromagneticparticles become superparamagnetic, therefore, sets a lower limit on thesize of particles which can be used in magnetic recording media. Formaximum recording density, ferromagnetic films should have the smallestparticles consistent with the above criteria. According to the prior artchemical processes used for producing ferromagnetic particles, thislimit of particle size has not been reached.

The ferromagnetic film, according to the present invention, utilizesferromagnetic particles having maximum dimensions in the range between50 and 500 Angstroms. The ferromagnetic particles are dispersed in thesurface layer of a substrate. The substrate material is selected so thatthe ferromagnetic material has low solubility therein. A preferredferromagnetic material is iron, since it has the highest magnetic momentper atom of all the elements. Other suitable ferromagnetic materials arenickel and cobalt. A preferred substrate material for use with iron iscopper, since iron has a solubility of about 1% in copper. The substratecan be bulk material or can be a layer as thin as 1,000 Angstroms, sincethe ferromagnetic properties are created in a layer near the surface. Toproduce a high density of iron particles of 200 Angstroms diameterrequires on the order of 2×10¹⁷ atoms/cm². Methods of fabricating theferromagnetic film are described in detail hereinafter.

One method of fabricating the ferromagnetic film includes ionimplantation of ferromagnetic ions into a substrate. In the exampledescribed above, pure iron ions are implanted into the surface layer ofcopper. As noted above, about 1% of the iron stays in solid solutionwith the copper. Excess iron forms a second phase particle of pure iron.Free oxygen present in the copper can oxidize the iron to form oxidessuch as FeO, Fe₂ O₃ and Fe₃ O₄. However, this is not a detrimentalresult, since Fe₂ O₃ and Fe₃ O₄ are ferromagnetic materials. Ionenergies in the range between 80 KeV and 120 KeV provide 200 to 500Angstroms range of iron ions in copper. The dose of iron ions is in therange between 5×10¹⁶ ions/cm² and 5×10¹⁷ ions/cm². Typically, the doseis about 2×10¹⁷ ions/cm². The implantation can be performed oncommercially available ion implanters such as the Varian/ExtrionDivision Model DF3000. Ions of the desired species are generated in anion source. An ion beam from the ion source is mass analyzed and focusedby an analyzer magnet and is then accelerated to the desired energy byan accelerator tube. The focused ion beam is deflected over the surfacearea of the target substrate to produce a uniform distribution ofimplanted ions. Ion implantation systems are generally known and are notwithin the scope of the present invention.

Heating of the copper substrate is required to permit growth of theparticles of iron and iron oxide to the desired size. Without heattreatment after ion implantation, the ferromagnetic particles can be sosmall that superparamagnetic properties are exhibited. The particlegrowth can be predicted from the temperature and duration of the heattreatment. A typical heat treatment is 500° C. for approximately 30minutes. Alternatively, the substrate can be allowed to increase intemperature during the implant, so that the particles grow as the ionsare implanted. However, this is likely to be less uniform and lesscontrolled than a known heat treatment.

In an alternative method for producing the ferromagnetic film, inaccordance with the present invention, a thin film of iron is depositedon a substrate of copper. The iron film can be deposited by evaporationor sputtering and should be approximately 200 to 500 Angstroms thick.The copper layer should be at least 1,000 Angstroms thick. As notedabove, the copper substrate can be a bulk material or can be a layer ofcopper adhered to another suitable substrate. A process known as ionbeam mixing is then applied to the iron film. An ion beam of an inertmaterial, such as argon, is applied to the iron film. Other suitablebeams are xenon and krypton. The ion beam must have sufficient energy topenetrate the iron film and the iron-copper interface and to drive theiron atoms into the underlying copper substrate. The ion beam causes theiron to be mixed into the near surface region of the copper substratewith a result similar to that produced by direct ion implantation ofiron. The required ion energy is about 40 KeV for argon and 130 KeV forxenon, while the required dose is in the range between 5×10¹⁶ ions/cm²and 5×10¹⁷ ions/cm². Known sputtering techniques can be used to form aniron film while the ion beam mixing can be performed by commercial ionimplanters, as noted above. After ion beam mixing, the substrate isheat-treated to form particles, as described hereinabove, in connectionwith direct ion implantation.

EXAMPLE

Copper foil 0.25 millimeter thick and 0.99999 pure was cut into disksamples about 3 millimeters in diameter. The samples weremetallurgically cleaned and polished and were mounted on a holder whichclamped the samples at their edges. The holder, which included verticaland horizontal shields to block the ion beam from selected samples, wasmounted in a Varian model DF4 ion implanter. The samples were implantedat a variety of doses in the range 1×10¹⁷ ions/cm² to 4×10¹⁷ ions/cm²(200 microamps ion beam current, 100 KeV, 5×10⁻⁶ Torr in theimplantation chamber). Samples were then heat treated in a vacuumfurnace at 1×10⁻⁶ Torr as follows:

Sample A

Dose: 4×10¹⁷ ions/cm²

Anneal: 500° C. for 2 hours

Sample B

Dose: 1×10¹⁷ ions/cm²

Anneal: 390° C. for 70 minutes

The samples were examined with a transmission electron microscope andfound to have a large amount of precipitates of iron oxides. Electrondiffraction ring patterns indicated the presence of FeO, Fe₂ O₃ and Fe₃O₄. Samples which had not been heat treated contained very smallcrystallites of less than 50 Angstroms. Sample A, which had been treatedat 500° C. for two hours, contained precipitates in the range of 50 to500 Angstroms.

While there has been shown and described what is at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

I claim:
 1. A ferromagnetic film comprising:a metallic substratematerial having low solubility for a selected ferromagnetic material;and a plurality of particles of said selected ferromagnetic material inthe surface layer of said substrate, said particles having a maximumdimension in the range between 50 and 500 Angstroms.
 2. A ferromagneticfilm as defined in claim 1 wherein selected ferromagnetic materialincludes iron and oxides of iron.
 3. A ferromagnetic film as defined inclaim 2 wherein said substrate comprises copper.
 4. A ferromagnetic filmas in claim 1 wherein said selected ferromagnetic material includesoxides of iron.